Print this pageAbstract: Carbon and Oxygen Isotope Analyses of Pleistocene Fluvial (Algal?) Pisolites and Pisolitic Caliche in Texas
13C/12C and 18O/16O ratios were determined for several samples of concretionary caliche from the Llano Estacado, northwestern Texas, and on pisoliths from Bexar County, south-central Texas, which are believed to be fluvial, possibly algal, in origin. ^dgr13C values for the caliche range from -0.6 to -6.3 PDB with an average of -3.7 for five samples. ^dgr18O values range from -4.0 to -5.1 SMOW with an average of -4.52. ^dgr13C values for the fluvial pisoliths range from -5.9 to -6.8 PDB with an average of -6.56 for six samples. ^dgr18O values range from -3.9 to -4.9 SMOW with an average of -4.38. These data indicate a considerable overlapping of values for postulated freshwater, algal pisoliths, and concretionary caliche. Evidently, isotopic differences cannot be used with certainty in determining the origin of some carbonate pisoliths as has been suggested by Magaritz, on the basis of studies in Israel. Magaritz obtained ^dgr13C values of between -9 and -11 PDB for caliches and a value of zero (0) for biogenic oncoliths.
The pisolitic gravels and conglomerates in Bexar County, Texas, are deduced to be fluvial on the basis of cross-bedding, cut-and-fill structures, the inclusion of freshwater gastropods and charophytes, and geographic distribution. Thin-section examination reveals a filamentous structure in some of the pisolith nuclei and laminations which may be algal in origin. Radioactive carbon dating indicates that the deposits are in excess of 50,000 years old. However, topographic location and fossil content indicate a Pleistocene age. A comparison of these pisolites with the concretionary caliche of the Llano Estacado and the nodular caliche of the Renosa Plateau of Southeast Texas disclosed virtually no common properties. Interestingly, some textural properties are very similar to those of Dun am's vadose pisoliths in the Permian Yates Formation of New Mexico.
AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA